The other problem is the prisoners dilemma: if the west stops having children and not-the-west does not stop then within a short time we will have replaced the west with an equivalent population breeding much faster for cultural or religious reasons.

There is also a world of social disruption, which could lead to environmentally disastrous wars, that come from those kinds of demographic inversions. The combination of worsening climate issues with those destabilizations might lead to acute depopulation at the global level, but not likely of a climate-friendly variety.

Destroying the world to improve some portion of the climate experienced by some portion of some population at some unspecified later date a century or more in the future versus, say, geoengineering and attacking the present-day cost structures of nuclear energy strikes me generally as a Bad Plan.

As long as we don't fill the gap with more immigration from poorer countries.

having less children if you live in a developed economy just means outsourcing your children to a developing economy.

If there are 1 million less children born in the US, it's very likely there will be 1 million more immigrants to take their place, and the places those immigrants come from will have higher birth rates to replace them

No skin in the game and less influence on the next generation having the skills and values needed to make the world better instead of carrying on the path it's on. Of course most people having kids aren't thinking of that aspect.